Philosophical skepticism comes in various forms. Radical forms of skepticism deny that knowledge or rational belief is possible and urge us to suspend judgment on many or all controversial matters. More moderate forms of skepticism claim only that nothing can be known with certainty, or that we can know little or nothing about the big question in life, such as whether God exists or whether there is an afterlife. Religious skepticism is "doubt concerning basic religious principles (such as immortality, providence, and revelation)".[4]Scientific skepticism concerns testing beliefs for reliability, by subjecting them to systematic investigation using the scientific method, to discover empirical evidence for them.

As a philosophical school or movement, skepticism arose both in ancient Greece and India. In India the Ajñana school of philosophy espoused skepticism. It was a major early rival of Buddhism and Jainism, and a major influence on Buddhism. Two of the foremost disciples of the Buddha, Sariputta and Moggallāna, were initially the students of the Ajñana philosopher Sanjaya Belatthiputta, and a strong element of skepticism is found in Early Buddhism, most particularly in the Aṭṭhakavagga sutra. Since skepticism is a philosophical attitude and a style of philosophising rather than a position, the Ajñanins may have influenced other skeptical thinkers of India such as Nagarjuna, Jayarāśi Bhaṭṭa, and Shriharsha.[5]

In Greece philosophers as early as Xenophanes (c. 570 – c. 475 BC) expressed skeptical views, as did Democritus[6]
and a number of Sophists. Gorgias, for example, reputedly argued that nothing exists, that even if there were something we could not know it, and that even if we could know it we could not communicate it.[7] The Heraclitean philosopher Cratylus refused to discuss anything and would merely wriggle his finger, claiming that communication is impossible since meanings are constantly changing.[8]Socrates also had skeptical tendencies, claiming that he knew nothing, or at least nothing worthwhile.[9]

There were two major schools of skepticism in the ancient Greek and Roman world. The first was Pyrrhonism, was founded by Pyrrho of Elis (c. 360–270 BCE). The second was Academic Skepticism, so-called because its two leading defenders, Arcesilaus (c. 315–240 BCE) who initiated the philosophy, and Carneades (c. 217–128 BCE), the philosophy's most famous proponent, were heads of Plato's Academy. Pyrrhonism's aims are psychological. It urges suspension of judgment (epoche) to achieve mental tranquility (ataraxia). The Academic Skeptics denied that knowledge is possible. The Academic Skeptics claimed that some beliefs are more reasonable or probable than others, whereas Pyrrhonian skeptics argue that equally compelling arguments can be given for or against any disputed view.[10] Nearly all the writings of the ancient skeptics are now lost. Most of what we know about ancient skepticism is from Sextus Empiricus, a Pyrrhonian skeptic who lived in the second or third century CE. His works contain a lucid summary of stock skeptical arguments.

Ancient skepticism faded out during the late Roman Empire, particularly after Augustine (354–430 CE) attacked the skeptics in his work Against the Academics (386 CE). There was little knowledge of, or interest in, ancient skepticism in Christian Europe during the Middle Ages. Interest revived during the Renaissance and Reformation, particularly after the complete writings of Sextus Empiricus were translated into Latin in 1569. A number of Catholic writers, including Francisco Sanches (c. 1550–1623), Michel de Montaigne (1533–1592), Pierre Gassendi (1592–1655), and Marin Mersenne (1588–1648) deployed ancient skeptical arguments to defend moderate forms of skepticism and to argue that faith, rather than reason, must be the primary guide to truth. Similar arguments were offered later (perhaps ironically) by the Protestant thinker Pierre Bayle in his influential Historical and Critical Dictionary (1697–1702).[11]

The growing popularity of skeptical views created an intellectual crisis in seventeenth-century Europe. One major response was offered by the French philosopher and mathematician René Descartes (1596–1650). In his classic work, Meditations of First Philosophy (1641), Descartes sought to refute skepticism, but only after he had formulated the case for skepticism as powerfully as possible. Descartes argued that no matter what radical skeptical possibilities we imagine there are certain truths (e.g., that thinking is occurring, or that I exist) that are absolutely certain. Thus, the ancient skeptics were wrong to claim that knowledge is impossible. Descartes also attempted to refute skeptical doubts about the reliability of our senses, our memory, and other cognitive faculties. To do this, Descartes tried to prove that God exists and that God would not allow us to be systematically deceived about the nature of reality. Many contemporary philosophers question whether this second stage of Descartes’ critique of skepticism is successful.[12]

In the eighteenth century a powerful new case for skepticism was offered by the Scottish philosopher David Hume (1711–1776). Hume was an empiricist, claiming that all genuine ideas can be traced back to original impressions of sensation or introspective consciousness. Hume argued forcefully that on empiricist grounds there are no sound reasons for belief in God, an enduring self or soul, an external world, causal necessity, objective morality, or inductive reasoning. In fact, he argued that "Philosophy would render us entirely Pyrrhonian, were not Nature too strong for it."[13] As Hume saw it, the real basis of human belief is not reason, but custom or habit. We are hard-wired by nature to trust, say, our memories or inductive reasoning, and no skeptical arguments, however powerful, can dislodge those beliefs. In this way, Hume embraced what he called a "mitigated" skepticism, while rejecting an "excessive" Pyrrhonian skepticism that he saw as both impractical and psychologically impossible.

Hume's skepticism provoked a number of important responses. Hume's Scottish contemporary, Thomas Reid (1710–1796), challenged Hume's strict empiricism and argued that it is rational to accept "common-sense" beliefs such as the basic reliability of our senses, our reason, our memories, and inductive reasoning, even though none of these things can be proved. In Reid's view, such common-sense beliefs are foundational and require no proof in order to be rationally justified.[14] Not long after Hume's death, the great German philosopher Immanuel Kant (1724–1804) argued that human moral awareness makes no sense unless we reject Hume's skeptical conclusions about the existence of God, the soul, free will, and an afterlife. According to Kant, while Hume was right to claim that we cannot strictly know any of these things, our moral experience entitles us to believe in them.[15]

Today, skepticism continues to be a topic of lively debate among philosophers.[16]

Religious skepticism generally refers to doubting given religious beliefs or claims. Historically, religious skepticism can be traced back to Xenophanes, who doubted many religious claims of the time. Modern religious skepticism typically emphasizes scientific and historical methods or evidence, with Michael Shermer writing that skepticism is a process for discovering the truth rather than general non-acceptance.[clarification needed] For example, a religious skeptic might believe that Jesus existed while questioning claims that he was the messiah or performed miracles (see historicity of Jesus). Religious skepticism is not the same as atheism or agnosticism, though these often do involve skeptical attitudes toward religion and philosophical theology (for example, towards divine omnipotence). Religious people are generally skeptical about claims of other religions, at least when the two denominations conflict concerning some stated belief. Additionally, they may also be skeptical of the claims made by atheists.[17] The historian Will Durant writes that Plato was "as skeptical of atheism as of any other dogma".

A scientific or empirical skeptic is one who questions beliefs on the basis of scientific understanding and empirical evidence.

Scientific skepticism may discard beliefs pertaining to purported phenomena not subject to reliable observation and thus not systematic or testable empirically. Most scientists, being scientific skeptics, test the reliability of certain kinds of claims by subjecting them to a systematic investigation using some type of the scientific method.[18] As a result, a number of claims are considered as "pseudoscience", if they are found to improperly apply or ignore the fundamental aspects of the scientific method.

Professional skepticism is an important concept in auditing. It requires an auditor to have a "questioning mind", to make a critical assessment of evidence, and to consider the sufficiency of the evidence.[19]